feat: add exit shell error

src/error.rs

@@ -2,6 +2,7 @@ use std::process::Command;
 
 pub enum ShellError {
     EmptyLine,
+    Exit,
     NotFound(Command),
     ExecuteFailure(String),
 }

src/main.rs

@@ -25,6 +25,9 @@ fn main() -> Result<()> {
                 Err(ShellError::EmptyLine) => {
                     continue;
                 }
+                Err(ShellError::Exit) => {
+                    break;
+                }
                 Err(ShellError::NotFound(cmd)) => {
                     println!("{}: command not found", cmd.get_program().to_string_lossy())
                 }